We have completed two projects during the past year. The first is a collaborative project between my group and that of A. Lev-Tov (Hebrew University, Israel) that examined the role of a group of propriospinal interneurons in mediating the activation of the CPG by stimulation of sacrocaudal afferents. The interneurons were identified by back-filling the ventral funiculus (VF) with a fluorescent dye while sensory afferents and/or motoneurons were back-filled with different fluorescent markers. These experiments demonstrated that VGluT1 and VGIuT2-positive primary afferents made direct synaptic contacts with a subset of the neurons back-labeled from the ventral funiculus. Imaging of the neurons, retrogradely labeled with calcium-green dextran, revealed that sacral VF-interneurons deliver both a tonic and a rhythmic drive to the locomotor CPG in the lumbar segments. Identification of these neurons and their activity patterns is important because it provides a potential entry point into the circuitry of the lumbar CPG. We also established if the locomotor circuitry is abnormal in a mouse model of spinal muscular atrophy. If it was, it could account for the profound weakness exhibited by these mice. However, we found that locomotor-like activity could be induced in the mutant mice and its characteristics were similar to those observed in wild type animals. This result suggests that the weakness seen in these animals is not due to dysfunction of the locomotor central pattern generator. We have recently begun using optogenetics to investigate the function of several different neuronal classes in the neonatal spinal cord including motoneurons. We have successfully performed preliminary experiments demonstrating the efficacy of light in silencing the cells expressing the hyperpolarizing light channel Archaerhodopsin-3. Further experiments are in progress to establish the effects of hyperpolarizing specific neuronal populations during locomotor-activity generated by the isolated mouse cord.